Conventionally, a coal-fired boiler discharges combustion exhaust gas, produced by coal combustion, from a furnace. This combustion exhaust gas contains fly ash and coal ash (solid particles) called high-porosity large-diameter ash (hereinafter referred to as “large-diameter ash”), with the details varying depending on the type of coal.
Of these ashes, fly ash is extremely fine particles with a particle size in the order of several micrometers. By comparison, large-diameter ash is relatively large particles with a particle size of about 1 mm or more, but has a low apparent specific gravity due to its high porosity.
A solid-gas two-phase stream discharged from a furnace 2 of a coal-fired boiler 1 passes through a flue 10, which is formed, for example, by an iron-sheet duct as shown in FIG. 9, and is subjected to necessary treatment, such as denitrification, before being released into the atmosphere through a funnel etc. (not shown).
In the flue 10 shown in FIG. 9, a first horizontal flue section 11, a first vertical flue section 12, a second horizontal flue section 13, a second vertical flue section 14, a third horizontal flue section 15, and a third vertical flue section 16 are provided sequentially from the upstream side in the flow direction of combustion exhaust gas, i.e., from the side of the furnace 2. Reference sign G in the drawings represents the flow of combustion exhaust gas, and 50 represents large-diameter ash.
In the configuration example shown in FIG. 9, a first hopper 20 and a second hopper 30 are installed at the lower ends of the first vertical flue section 12 and the second vertical flue section 14, respectively, to collect large-diameter ash scattered from the furnace 2. In addition, a denitrification apparatus 40, which denitrifies combustion exhaust gas passing therethrough, is installed in the third vertical flue section 16. The first hopper 20 installed at the lower end of the first vertical flue section 12 includes an inclined surface 21 of which the angle is set such that collected large-diameter ash falls. However, being an iron-sheet wall surface as with the duct, the conventional inclined surface 21 has a high coefficient of restitution, so that large-diameter ash rebounding far off the inclined surface 21 has a high probability of jumping over the first hopper 20 and scattering to the second horizontal flue section 13.
In particular, since the flow velocity of combustion exhaust gas is higher at the center of the cross-section of the flue 10, large-diameter ash having jumped over the first hopper 20 and joined the fast stream of combustion exhaust gas is highly likely to reach the denitrification apparatus 40 without being collected in the second hopper 30, either, due to its low apparent specific gravity.
For example, the denitrification apparatus 40 has a configuration in which a denitrification agent (denitrification catalyst) of vanadium dioxide supported on a lattice-shaped titanium oxide carrier is placed in a pallet, and a large number of this pallet are disposed inside the apparatus. Thus, combustion exhaust gas, which is a solid-gas two-phase stream, is denitrified while passing through the denitrification apparatus 40, but at the same time the lattice-shaped denitrification catalyst is clogged with large-diameter ash passing therethrough along with the combustion exhaust gas.
To prevent such clogging of a denitrification catalyst, a common practice is to install a metal-mesh trapping screen S inside the flue on the downstream side from the first hopper 20, as shown in PTL 1 and PTL 2, for example.